1. Field of the Invention
The present invention relates to an in-line pipe inspection tool, also known as a pipeline pig.
2. Summary of the Prior Art
It is known to inspect the inside of a pipeline using a pipeline pig which passes down the pipe. For magnetic inspection, the pig has permanent magnets defining first and second pole pieces, which are positioned adjacent the inner wall of the pipe. Those magnets then generate magnetic fields which magnetise the wall of the pipe. Sensors are provided between the magnetic poles, which detect the magnetic flux density at the internal surface of the pipe. The magnetic field in the pipe wall is normally constant, but is disturbed by a flux changing feature, such as a defect, weld bead or wall thickness change, and magnetic flux then leaks out of the pipe at such a feature, to be detected by the sensors. As the pipeline pig is driven along the pipe, the location of the pole pieces, and the sensors, moves along the pipe enabling the internal surface of the pipe to be inspected.
In most pipeline pigs, the pole pieces extend cylindrically around the pig, to define a fixed structure. However, such an arrangement has the problem that it cannot then adapt to pipelines of different diameters, and to other features of pipeline such as mitred bends, valves, etc.
Therefore, it has been proposed to mount the pole pieces, sensors, etc on an inspection platform which is connected to the rest of the pig by a variable linkage. An example of such a pig is disclosed in U.S. Pat. No. 6,538,431. In that document, a pipeline pig has a central body member with inspection platforms connected thereto by links which have carrier members on the central body, which carrier members are axially slideable along the body, and which sliding is resisted by springs. The springs thus bias the carrier members to a predetermined position, and the linkage then holds the inspection platforms with a predetermined spacing from the central body. However, if inward radial pressure is applied to the inspection platforms, e.g. because of a reduction in the diameter of the pipe, the carrier members are pushed against the resistance of the springs thereby permitting the shape of the linkage to change, and to permit the inspection platform to move inwardly.
Thus, U.S. Pat. No. 6,538,431 proposes passive movement of the inspection platforms. Since the inspection platforms must make contact with the pipe wall (both as part of their inspection function but also when they are to be moved radially), U.S. Pat. No. 6,538,431 also proposed that the inspection platforms have bristles thereon, to act as magnetic coupling between the pole pieces and the pipe wall, and also to provide a contact for the inward radial force to be applied.
However, the use of such bristles has the disadvantages that they cause friction between the inspection platform and the wall of the pipe, thereby requiring a high force to move the pipeline pig along the pipe.